NG_ASYNC(4) BSD Kernel Interfaces Manual NG_ASYNC(4)NAMEng_async — asynchronous framing netgraph node type
SYNOPSIS
#include <netgraph/async/ng_async.h>
DESCRIPTION
The async node type performs conversion between synchronous frames and
asynchronous frames, as defined for the PPP protocol in RFC 1662. Asyn‐
chronous framing uses flag bytes and octet-stuffing to simulate a frame
oriented connection over an octet-oriented asynchronous serial line.
The node transmits and receives asynchronous data on the async hook.
Mbuf boundaries of incoming data are ignored. Once a complete packet has
been received, it is decoded and stripped of all framing bytes, and
transmitted out the sync hook as a single frame.
Synchronous frames are transmitted and received on the sync hook. Pack‐
ets received on this hook are encoded as asynchronous frames and sent out
on async. Received packets should start with the address and control
fields, or the PPP protocol field if address and control field compres‐
sion is employed, and contain no checksum field. If the first four bytes
are 0xff 0x03 0xc0 0x21 (an LCP protocol frame) then complete control
character escaping is enabled for that frame (in PPP, LCP packets are
always sent with no address and control field compression and all control
characters escaped).
This node supports “flag sharing” for packets transmitted on async. This
is an optimization where the trailing flag byte of one frame is shared
with the opening flag byte of the next. Flag sharing between frames is
disabled after one second of transmit idle time.
HOOKS
This node type supports the following hooks:
async Asynchronous connection. Typically this hook would be connected
to a ng_tty(4) node, which handles transmission of serial data
over a tty device.
sync Synchronous connection. This hook sends and receives synchronous
frames. For PPP, these frames should contain address, control,
and protocol fields, but no checksum field. Typically this hook
would be connected to an individual link hook of a ng_ppp(4) type
node.
CONTROL MESSAGES
This node type supports the generic control messages, plus the following:
NGM_ASYNC_CMD_GET_STATS
This command returns a struct ng_async_stat containing node statis‐
tics for packet, octet, and error counts.
NGM_ASYNC_CMD_CLR_STATS
Clears the node statistics.
NGM_ASYNC_CMD_SET_CONFIG
Sets the node configuration, which is described by a struct
ng_async_cfg:
struct ng_async_cfg {
u_char enabled; /* Turn encoding on/off */
u_int16_t amru; /* Max receive async frame len */
u_int16_t smru; /* Max receive sync frame len */
u_int32_t accm; /* ACCM encoding */
};
The enabled field enables or disables all encoding/decoding func‐
tions (default disabled). When disabled, the node operates in sim‐
ple “pass through” mode. The amru and smru fields are the asynchro‐
nous and synchronous MRU (maximum receive unit) values, respec‐
tively. These both default to 1600; note that the async MRU applies
to the incoming frame length after asynchronous decoding. The accm
field is the asynchronous character control map, which controls the
escaping of characters 0x00 thorough 0x1f (default 0xffffffff).
NGM_ASYNC_CMD_GET_CONFIG
This command returns the current configuration structure.
SHUTDOWN
This node shuts down upon receipt of a NGM_SHUTDOWN control message, or
when all hooks have been disconnected.
SEE ALSOnetgraph(4), ng_ppp(4), ng_tty(4), ngctl(8)
W. Simpson, PPP in HDLC-link Framing, RFC 1662.
W. Simpson, The Point-to-Point Protocol (PPP), RFC 1661.
HISTORY
The ng_async node type was implemented in FreeBSD 4.0.
AUTHORS
Archie Cobbs ⟨archie@FreeBSD.org⟩
BSD January 19, 1999 BSD
